Glaze polishing polycrystalline alumina material

ABSTRACT

Increased optical transmission is provided for a body of optically transparent polycrystalline alumina by coating at least one major surface of said alumina body with a solid glaze coating and subsequently melting said coating at elevated temperatures to dissolve the surface alumina grains in physical content therewith to provide a smoother surface when the coating residue has been removed. More particularly, a solid glaze coating is first deposited on said major surface of the alumina body and the coated alumina body thereafter heated to sufficiently elevated temperatures for the glaze liquification whereupon surface alumina grains in physical contact therewith are dissolved in a manner which reduces the high spots on the individual surface alumina grains without materially introducing low spots at the grain boundaries and wherein the individual alumina surface grains have been preferentially etched in certain of the crystalline axis directions. The present glaze material comprises an alkali metal borate other than lithium borate having an alkali metal oxide to boron oxide weight ratio in the range from about 1:2 up to 1:4 and which is deposited on the major surface of said alumina body as a liquid suspension.

This is a division of application Ser. No. 666,858, filed Oct. 31, 1984,now U.S. Pat. No. 4,633,137.

BACKGROUND OF THE PRESENT INVENTION

The present invention relates to an optically transparent body of highdensity, polycrystalline alumina wherein at least one of the majorsurfaces is polished with a particular type molten fluxing agent forincreased optical transmission. A tubular form of the glaze polishedmaterial when used as the light transmissive envelope for an improvedhigh intensity electric discharge lamp provides a higher light outputthan is conventionally obtained with the unpolished material. Suchimproved envelope material is particularly useful in high intensitysodium vapor lamp constructions utilizing a grade of the polycrystallinealumina which already exhibits superior optical transmission in theunpolished condition as compared with earlier polycrystalline aluminaceramic.

The original polycrystalline alumina material made according to U.S.Pat. No. 3,026,210 (Coble), assigned to the assignee of the presentinvention, is still widely used for the light transmissive envelope inhigh intensity electric discharge lamps. This polycrystalline aluminaceramic is characterized by a relatively uniform large grain size andcan be prepared with a minimal of secondary phase magnesia-aluminaspinel at the grain boundaries. In U.S. Pat. No. 4,150,317, alsoassigned to the present assignee, there is disclosed a further improvedoptical grade of said alumina ceramic consisting essentially ofrelatively uniform equiaxed grains of alumina containing no more thanabout 150 ppm magnesium with essentially no porosity and secondaryphase. Said material exhibits increased in-line transmission and isfurther characterized by the substantial absence of grain-growthpromoting impurities and a pore volume fraction no greater than about10⁻². The average grain size of said material is approximately 26microns diameter with substantially all grains having an averagediameter in the range extending from about 20 microns diameter up toabout 35 microns diameter. Whereas the original Coble material containeda small but effective amount up to about 0.5 weight percent magnesiacontent, a reduction in the magnesia content was found to impart greatercontrol over grain growth as well as pore removal for less lightscattering in the sintered product. A still more recently issued U.S.Pat. No. 4,285,732, also assigned to the assignee of the presentinvention, discloses a further improved grade of said opticallytransparent ceramic consisting essentially of alumina, magnesium and acomponent select from the group consisting of zirconium, hafnium andmixtures thereof. Magnesium is present in said further improved aluminamaterial at amounts equivalent to from about 0.03% MgO by weight to lessthan about 0.15% MgO by weight of said sintered material, zirconiumbeing present at amounts equivalent to from higher than about 0.002%ZrO₂ by weight up to about 0.07% ZrO₂ by weight of said sinteredmaterial, and hafnium being present at amounts equivalent to from higherthan about 0.003% HfO₂ up to about 0.12% HfO₂ by weight of the sinteredmaterial. Mixtures of these additives are said to comprise all ratios ofzirconium and hafnium in amounts equivalent to from higher than 0.002%by weight up to about 0.12%.

It is also known to improve optical transmission in the foregoingpolycrystalline alumina materials by a flux polishing treatment astaught in U.S. Pat. Nos. 3,935,495; 4,033,743; and 4,079,167, also allassigned to the present assignee. The unpolished ceramic material isimmersed in a molten inorganic flux selected from the group consistingof alkali metal salts and binary oxide systems having an alkali metaloxide constituent which dissolves the surface alumina grains andproduces a relatively smooth surface appearance. In said treatment, thehigh spots of the individual surface alumina grains are reduced withoutmaterially etching the grain boundaries and the flux residue is removedfrom the treated ceramic generally by acid washing at near ambienttemperatures. This selective etching process is carried out in themolten inorganic flux bath at temperatures up to around 1000° C. withmore elevated temperatures producing undesirable grain boundary etchingaccompanied by excessive volatilization of the fluxing agent being used.The preferred flux material is a eutectic composition of sodium boratesalt at molar ratios between two moles B₂ O₃ up to four moles B₂ O₃ perone mole Na.sub. 2 O. Certain difficulties have been experienced withsaid prior art molten flux treatment, however, including a need tocontain the molten flux in platinum vessels along with contamination ofthe molten flux by surface impurities contained in the alumina bodiesbeing polished. The latter problem makes it difficult to control thetreatment process while having to conduct said process in a container ofthe molten flux is not adaptable to using automated handling equipmentfor this task.

It is desirable, therefore, to carry out the flux polishing treatment ina manner avoiding the foregoing difficulties. By eliminating a moltenflux bath for said treatment, it is further possible to reclaimpolycrystalline alumina arc tubes which have been contaminated duringthe manufacture of high pressure sodium vapor lamps. It would also bedesirable to polish polycrystalline alumina arc tubes by treating onlythe exterior major surface and thereby avoid having to remove fluxresidue from the tube interior. The polishing can also be used to alterthe physical dimensions of alumina parts being fitted together invarious product applications.

SUMMARY OF THE INVENTION

It has now been discovered that said polycrystalline alumina bodies canbe polished with a particular glaze coating for improved opticaltransmission. More particularly, a solid glaze coating is firstdeposited on one or more major surfaces of the alumina body and thecoated body thereafter heated to elevated temperatures of at least 800°C. which melts this coating and dissolves the surface alumina grains inphysical contact therewith. The coating residue is thereafter removedfrom the surface of the treated ceramic by conventional means to provideoptical transmission improvement comparable with that produced by moltenflux bath immersion. Specifically, the present method of increasingoptical transmission for an already optically transparent body ofsintered polycrystalline alumina comprises:

(a) depositing a solid glaze coating on at least one major surface ofsaid alumina body which dissolves the surface alumina in physicalcontact therewith when subsequently melted at elevated temperatures,said solid glaze coating comprising an alkali metal borate other thanlithium borate having an alkali metal oxide to boron oxide weight ratioin the range from about 1:2 up to 1:4,

(b) heating said coated alumina body to elevated temperatures of atleast about 800° for a sufficient time period to liquify the glazecoating and dissolve the surface alumina grains,

(c) removing the coating residue from the surface of said alumina body.

A lithium borate glaze is unsuitable in the present method by reason ofproducing relatively insoluble crystals on the treated surface whichcannot thereafter be removed during the acid washing step. The solidglaze coating is conveniently deposited on the alumina surface from aliquid suspension or solution. An especially preferred method ofdepositing the solid glaze coating employs a preformed frit of thealkali metal borate fluxing agent suspended in an organic liquid. Sincethis preformed frit remains somewhat water-soluble, the employment of anorganic liquid as the suspending medium avoids loss of the frit from thesurface deposit. A drawback experienced with depositing the solid glazecoating from a liquid solution of the glaze forming oxides is a lessersolid coating thickness which may not prove adequate in subsequentlyremoving all contaminated surface alumina. Said drawback with a liquidsolution deposition of the glaze material renders the treatment processmore difficult to control. On the other hand, the term "glaze" as usedin the present specification denotes either a solid coating of thevitreous fluxing agent or a solid coating of the powdered batch materialused to produce said vitreous fluxing agent. Increasing the viscosity ofsaid vitreous fluxing agent when melted to carry out the presentpolishing method is also desirable. Up to about 20 weight percent SiO₂in the glaze material serves this purpose.

The present treatment produces an optically transparent sinteredpolycrystalline alumina body having at least one glaze polished majorsurface and wherein the individual alumina surface grains have beenpreferentially etched in certain crystalline axis directions.Accordingly while said method still dissolves alumina at the surface ofthe polycrystalline alumina body being treated in a manner producingsurface smoothness, there is preferential etching of the alumina grainsin at least one of the crystalline axis directions. This type ofpreferential etching although still dissolving the surface layer ofalumina grains rather than dissolving material at the grain boundariesproduces less etching of those surface alumina grains which are notphysically aligned in the preferred crystalline axis direction. Certaindark spots appearing in microphotographs of the glaze polished aluminasurface represent alumina grains protruding from an otherwise flatpolished surface. As previously indicated, a polycrystalline aluminabody treated in accordance with the present invention may comprise highpurity alumina containing a small but effective amount up to about 0.5weight percent magnesia or consist essentially of relatively uniformequiaxed grains of alumina containing no more than about 150 ppmmagnesium and having essentially no porosity or secondary phase. Thelatter alumina material also has an average grain size of approximately26 microns diameter with substantially all grains having an averagediameter in the range extending from about 20 microns diameter up toabout 35 microns diameter. Another preferred alumina composition whichcan be polished in accordance with the present method consistsessentially of alumina, magnesium and a component selected from thegroup consisting of zirconium hafnium and a mixture thereof in relativeproportions above indicated.

An illustrative high intensity discharge lamp according to the presentinvention employs a glaze polished polycrystalline alumina arc tube asthe light transmissive envelope. Said improved lamp constructionincludes electrodes being sealed into the ends of said lighttransmissive envelope which also contains an ionizable medium forproducing said discharge. The present improvement resides in utilizingas the light transmissive envelope a tube consisting essentially of anoptically transparent sintered polycrystalline alumina having at leastone major surface glaze polished so as to reduce the high spots on theindividual surface alumina grains without materially introducing lowspots at the grain boundaries for increased optical transmission uponglaze removal and wherein the individual alumina surface grains havebeen preferentially etched in certain of the crystalline axisdirections. In accordance with said improvement, the glaze polishingtreatment increases optical transmission of an arc tube by up to about20% or greater in total transmission and with the lumen output of arepresentative high pressure sodium vapor lamp employing said arc tubeexhibiting similar improvement.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of a jacketed high pressure sodium vapor lampemploying the improved glaze polished polycrystalline alumina materialof the present invention;

FIG. 2 is an enlarged detailed view of one end closure employed in theFIG. 1 lamp embodiment;

FIG. 3 is an enlarged detailed view of the remaining end closureemployed in said FIG. 1 lamp embodiment; and

FIG. 4 is a microphotograph at 120 times magnification illustratingglaze polished polycrystalline alumina material according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A typical high pressure sodium vapor discharge lamp utilizing thepresent improvement is illustrated in FIG. 1. Said lamp 1 corresponds toa 400-watt size lamp contained within an outer vitreous envelope orjacket 2 of elongated ovoid shape. Said vitreous outer envelope 2 has astandard mogul screw base 3 attached to one end and comprises areentrant stem press seal 4 through which extends, in conventionalfashion, a pair of relatively heavy lead-in conductors 5 and 6 whoseouter ends are connected to the screw base shell 7 and eyelet 8 of thebase. The inner envelope or arc tube 9 centrally located within saidouter envelope comprises a length of light-transmitting polycrystallinealumina ceramic tubing which has been glaze polished according to thepresent method. The upper end of said arc tube is sealed by closuremeans 10 more fully described in another U.S. Pat. No. 4,034,252,assigned to the assignee of the present invention, through which extendsa niobium lead wire 11 hermetically sealed thereto. The inlead supportsthe upper electrode which may be generally similar to the lowerelectrode and with both of said electrodes being more specificallydescribed in the following FIG. 2-3 embodiments. The external portion ofinlead 11 passes through a loop 12 in transverse support wire 13 whichis attached to side rod 14. This arrangement permits thermal expansionof the arc tube during lamp operation when the lower end seal is rigidlyfixed in place, and the resilient metal ribbon 15 assures goodelectrical connection. Side rod 14 is welded to lead-in conductor 6 andhas its upper end braced by spring clamp 16 which engages invertednipple 17 in the dome end of the outer vitreous envelope. The lower endclosure 20 for said lamp has a central aperture through which extends athin walled niobium tube 21 serving both as an exhaust tube and as aninlead. Said tube extends but a short distance through the end closureand is hermetically sealed therethrough by sealing frit as will befurther explained in the description given for the FIG. 3 embodimentdealing with said end closure. The closed arc tube is supported in theouter envelope by connector 22 which is welded across from tubularinlead 21 to a support rod 23 joined to lead-in conductor 5.

Upper ceramic closure 10 for the above described lamp embodiment detailis shown in FIG. 2. Specifically, said ceramic closure 10 comprises aceramic plug 24 which is entirely inserted into one end of the arc tubeand has a metal cross piece 26, which can be a short wire length, joinedto in-lead 11 at the exterior surface 28 of said ceramic plug. Saidcross piece is bent or arched slightly about its mid point in ahorizontal plane transverse to said in-lead in order to have its endsengage the edges of the arc tube and physically support the thermionicelectrode joined thereto. A hermetic seal 29 is formed in the annularrecess between the assembled ceramic plug and inner wall of the arc tubeas hereinafter further explained. A second hermetic seal 30 is formed atthe central opening 32 in ceramic plug 24 and which extends to a ridgeof sealing frit 34 on the surface of said plug reaching up to the crosspiece 26. All hermetic sealing in the illustrated closure embodiment canbe provided with a measured quantity of sealing glass frit being placedon the ceramic plug, suitably as a slurry and the closure assembly thenbeing heated to the melting point of the glass frit. The molten frit isdrawn by capillary action into the annular free space between the arctube and ceramic plug as well as into the free space formed between thecentral opening in said plug with the electrode in-lead. Said in-lead 11is joined to tungsten shank 36 of a conventional thermionic electrode 38having one or more refractory metal coils wound around and which isfurther described in the aforementioned U.S. Pat. No. 4,034,252.

The lower ceramic closure 20 for the above lamp embodiment is shown inFIG. 3. Specifically, said improved ceramic enclosure 20 comprisesassembly of a ceramic disk 40 having a larger diameter than the innerdiameter of said arc tube 9 which is hermetically sealed with ceramicsealing frit at one end of said arc tube as well as the outermostsurface 42 of a ceramic plug 44 which has been inserted into the end ofsaid arc tube. The butt-sealed interface between said ceramic disk 40and ceramic plug 44 is formed with a ring or washer 46 of ceramic fritmaterial which further provides a hermetic seal 48 to associatedelectrode structure 50. Said conventional electrode 50 is shown toinclude part of a metal exhaust tube and in-lead member 21 that isinserted through a central opening 52 in both said ceramic disk andceramic plug for hermetic sealing after assembly of said componentparts. As can be noted, hermetic sealing 48 between the electrode andceramic components is generally limited to sealing engagement with theplug member only.

Regarding the present departure from the prior art method of fluxpolishing as taught in the aforementioned U.S. Pat. No. 3,935,495,representative samples of unpolished polycrystalline alumina tubing weresubjected to both polishing treatments for comparison of the relativedegree of improvement in optical transmission attributable thereto. Alltubing samples used in said comparison consisted of alumina tubingcontaminated with surface impurities which reduced light transmissiontherethrough. One group of said alumina tubing samples was polished byimmersion in a molten bath of alkali metal borate under conditionsproducing an approximately 50 micron thickness removal of the surfacealumina grains. The remaining group of said tubing samples was glazepolished twice with an aqueous solution of borax, potassium carbonateand boric oxide which resulted in approximately 25 microns thicknessremoval of the surface alumina grains during each polishing treatment.The total optical transmission improvement produced with both polishingtreatment is reported in Table I below.

                  TABLE I                                                         ______________________________________                                        Type Tubing    Total Transmittance (%)                                        ______________________________________                                        Unpolished     78.3                                                           Immersion Polished                                                                           94.2                                                           Glaze Polished 94.3                                                           ______________________________________                                    

As can be noted from the Table I results, comparable improvement intotal optical transmission can be achieved in accordance with thepresent invention without contaminating a molten flux both medium as wasexperienced in the prior art method. It can also be appreciated that asimilar improvement would result in the decontamination ofpolycrystalline alumina arc tubes which have become contaminated duringassembly in high intensity electric discharge lamps.

To still further illustrate the nature and degree of the presentimprovement as applicable to said high intensity electric dischargelamps, another comparison was made wherein 400 watt size high pressuresodium vapor test lamps were constructed employing polycrystallinealumina arc tubes before and after the glaze polishing treatment.Accordingly, a first group of said test lamps employed unpolished arctubes having the optical transmittance reported in the following TableII whereas a second group of said test lamps utilized arc tubes from thesame manufacturing lot which had been polished twice with theaforementioned glaze composition to provide the optical transmittanceimprovement also reported in said Table. The lumen gain experienced forthe test lamps constructed in said manner is also reported in said Tableto confirm that the benefit impacted to the arc tubes by glaze polishingis reflected in the lamp operation.

                  TABLE II                                                        ______________________________________                                                     Total         Lamp LPW                                           Type Arc Tube                                                                              Transmittance (%)                                                                           (100 Hours)                                        ______________________________________                                        Unpolished   75.0          102.7                                              Glaze Polished                                                                             93.5          124.6                                              ______________________________________                                    

As can be noted from the foregoing test results, the improved lumen perwatt values attributable to glaze polishing exceeded 20% as did theimproved optical transmittance responsible for this improvement. Adetailed example is provided below illustrating preparation of thepreformed frit that represents an especially preferred glaze compositionfor use according to the present invention. Also included in saidexample are representative conditions for such use of the fritcomposition.

EXAMPLE

Approximately 119 grams of borax, were mixed with 87 grams boric oxide,13 grams silica, and 43 grams of potassium carbonate and this mixturethereafter melted in a platinum vessel at approximately 900° C. to forma glass material that was poured into water to produce a frit. The fritwas then ground to a powder that passed thru a 120 mesh size screen. Aliquid suspension of said frit material was prepared again in aconventional manner employing approximately 75 grams of the powder beingsuspended in approximately 56 grams iso-propanol, 3 grams of a polyvinylpyrrolidone binder, and 0.6 grams of a glycerol trioleate dispersant. Asolid coating of the suspended frit material was next deposited onunpolished polycrystalline alumina tubing by removal of the liquidsolvent under ordinary ambient drying conditions. The coated aluminatubing was next heated in air to temperatures in the range 875°-1025° C.for 30 minutes to 2 hours in order to melt said coating and remove theunderlying alumina surface. Temperatures above 1025° C. were avoided toprevent extensive volatilization of the frit coating which occurredafter 1 hour of exposure and which produced a rough and pitted aluminasurface. Flux residue was thereupon removed by washing the treatedtubing in a heated dilute aqueous HCl solution.

The appearance of a properly polished alumina surface in accordance withthe present invention is illustrated in the accompanying FIG. 3photomicrograph. As can be noted from said photograph, the polishedsurface is of a generally flat nature wherein most high spots on theindividual alumina particles have been removed without introducingsignificant low spots at the grain boundaries. The few dark spotsappearing in said photograph represent alumina grains protruding fromthe otherwise flat polished surface.

It will be apparent from the foregoing description that variousmodifications can be employed in the present polishing method withoutdeparting from the spirit and scope of the present invention. Forexample, various minor additions can be made to the particular aluminamaterials above specifically described without altering the ability ofsaid material to be glaze polished in accordance with the presentinvention. Additionally, the glaze material can be modified other thanabove specifically illustrated with minor additions of other additivesto further enhance rheological characteristics during the treatmentsteps. It is intended to limit the present invention, therefore, only byscope of the following claims.

We claim:
 1. A method of increasing the optical transmission of anoptically transparent body of sintered polycrystalline alumina whichcomprises:(a) depositing a solid glaze coating on at least one majorsurface of said alumina body which dissolves the surface alumina inphysical contact therewith when subsequently melted at elevatedtemperatures, said solid glaze coating comprising an alkali metal borateother than lithium borate having an alkali metal oxide to boron oxideweight ratio in the range from about 1:2 up to about 1:4, (b) heatingsaid coated alumina body to elevated temperatures of at least about 800°C. for a sufficient time period to liquify the glaze coating anddissolve the surface alumina grains, and (c) removing the coatingresidue from the surface of said alumina body.
 2. A method as in claim 1wherein the glaze coating comprises a liquid suspension of oxidesforming the glaze composition.
 3. A method as in claim 2 wherein saidoxides comprise a mixture of borax, boron oxide, and potassiumcarbonate.
 4. A method as in claim 1 wherein the glaze coating comprisesa liquid suspension of a glass frit.
 5. A method of increasing theoptical transmission of an unpolished optically transparent body ofsintered polycrystalline alumina which comprises:(a) forming a glassfrit comprising an alkali metal borate other than lithium borate havingan alkali metal oxide to boron oxide weight ratio in the range fromabout 1:2 up to about 1:4, (b) depositing a solid glaze coating with aliquid suspension of said glass frit on at least one major surface ofsaid alumina body and removing the liquid from said coating, (c) heatingsaid coated alumina body to elevated temperatures of at least about 800°C. for a sufficient time period to liquify the glaze coating anddissolve the surface alumina grains in physical contact therewith, and(d) removing the coating residue from the surface of said alumina body.6. A method as in claim 5 wherein the liquid suspension employs anorganic liquid.
 7. A method as in claim 5 wherein the coating residue isremoved by acid washing.